1. Field of the Invention
The present invention is directed generally to an exposure method and an exposure apparatus, and more particularly, to a projection exposure method and a projection exposure apparatus which are employed in a lithography process for liquid crystal elements and semiconductor memory cells having regular hyperfine patterns.
2. Related Background Art
A method of transferring mask patterns on a substrate typically by the photolithography method is adopted in manufacturing semiconductor memories and liquid crystal elements. In this case, the illumination light such as ultra-violet rays for exposure strikes on the substrate having its surface formed with a photosensitive resist layer through a mask formed with the mask patterns. The mask patterns are thereby photo-transferred on the substrate.
The typical hyperfine mask patterns of the semiconductor memory and the liquid crystal element can be conceived as regular grating patterns arrayed vertically or horizontally at equal spacings. Formed, in other words, in the densest pattern region in this type of mask patterns are the grating patterns in which equally-spaced transparent lines and opaque lines, formable on the substrate, for attaining the minimum line width are arrayed alternately in X and/or Y directions. On the other hand, the patterns having a relatively moderate degree of fineness are formed in other regions. In any case, the oblique patterns are exceptional.
Besides, a typical material for the photosensitive resist exhibits a non-linear photosensitive property. A chemical variation thereof quickly advances on giving an acceptance quantity greater than a certain level. If smaller than this level, however, no chemical variation advances. Hence, there exists a background wherein if a difference in light quantity between a light portion and a shade portion is sufficiently secured with respect to a mask pattern projected image on the substrate, a desired resist image according to the mask patterns can be obtained even when a boundary contrast between the light portion and the shade portion is somewhat low.
In recent years, a projection exposure apparatus such as a stepper, etc. for transferring the mask pattern on the substrate by reductive projection has been often employed with a hyperfiner pattern construction of the semiconductor memory and the liquid crystal element. Special ultra-violet rays having a shorter wavelength a narrower wavelength distributing width are employed as illumination light for exposure. The reason why the wavelength distribution width is herein narrowed lies in a purpose for eliminating a deterioration in quantity of the projected image due to a chromatic aberration of the projection optical system of the projection exposure apparatus. The reason why the shorter wavelength is selected lies in a purpose for improving the contrast of the projected image. Shortening of the wavelength of the illumination light induces a limit in terms of constraints of lens materials and resist materials in addition to the fact that no appropriate light source exists for the much hyperfiner mask patterns required, e.g., for the projection exposure of line widths on the submicron order. This is the real situation.
In the hyperfine mask patterns, a required value of the pattern resolution line width is approximate to the wavelength of the illumination light. Hence, it is impossible to ignore influences of diffracted light generated when the illumination light penetrates the mask patterns. It is also difficult to secure a sufficinet light-and-shade contrast of the mask pattern projected image on the substrate. In particular, the light-and-shade contrast at the pattern line edges remarkably declines.
More specifically, respective diffracted light components a 0th-order diffracted light component, (+) primary diffracted light components and those greater than (+) secondary diffracted light components that are generated at respective points on the mask patterns due to the illumination light incident on the mask from above-pass through the projection optical system. These light components are converged again at the respective points on the substrate conjugate these points, thereby forming the image. However, the (+) primary diffracted light components and those larger than the (+) secondary diffracted light components have a much larger diffraction angle than that of the 0th-order diffracted light component with respect to the hyperfiner mask patterns and are therefore incident on the substrate at a shallower angle. As a result, a focal depth of the projected image outstandingly decreases. This causes such a problem that a sufficient exposure energy can not be supplied only to some portions corresponding to a part of thickness of the resist layer.
It is therefore required to selectively use the exposure light source having a shorter wavelength or the projection optical system having a larger numerical aperture in order to transfer the hyperfiner patterns. As a matter of course, a strive for optimizing both of the wavelength and the numerical aperture can be also considered. Proposed in Japanese Patent Publication No. 62-50811 was a so-called phase shift reticle in which a phase of the transmitted light from a specific portion among the transmissive portions of reticle circuit patterns deviates by xcfx80 from a phase of the transmitted light from other transmissive portions. When using this phase shift reticle, the patterns which are hyperfiner than in the prior art are transferable.
In the conventional exposure apparatus, however, it is presently difficult to provide the illumination light source with a shorter wavelength (e.g., 200 nm or under) than the present one for the reason that there exists no appropriate optical material usable for the transmission optical member.
The numerical aperture of the projection optical system is already approximate to the theoretical limit at the present time, and a much larger numerical aperture can not be probably expected.
Even if the much larger numerical aperture than at present is attainable, a focal depth expressed by +xcex/2NA2 is abruptly reduced with an increase of the numerical aperture. There goes conspicuous the problem that the focal depth needed for an actual use becomes smaller and smaller. On the other hand, a good number of problems inherent in the phase shift reticle, wherein the costs increase with more complicated manufacturing steps thereof, and the inspecting and modifying methods are not yet established.
Disclosed, on the other hand, in U.S. Pat. No. 4,947,413 granted to T. E. Jewell et al is the projection lithography method by which a high contrast pattern projected image is formed with a high resolving power on the substrate by making the 0th-order diffracted light component coming from the mask patterns and only one of the (+) and (xe2x88x92) primary diffracted light components possible of interference by utilizing a spatial filter processing within the Fourier transform surface in the projection optical system by use of an off-axis illumination light source. Based on this method, however, the illumination light source has to be off-axis-disposed obliquely to the mask. Besides, the 0th-order diffracted light component is merely interfered with only one of the (+) and (xe2x88x92) primary diffracted light components. Therefore, the light-and-shade contrast of edges of the pattern image is not yet sufficient, the image being obtained by the interference due to unbalance in terms of a light quantity difference between the 0th-order diffracted light component and the primary diffracted light component.
It is a primary object of the present invention, which has been devised in the light of the foregoing problems, to attain the exposure with a high resolving power and large focal depth even when using an ordinary reticle by making the illumination light incident on a mask at a predetermined angle inclined to the optical axis of an illumination optical axis or a projection optical system, providing a member for making the illumination light incident obliquely on the mask in the illumination optical system and illuminating the mask without any loss in light quantity.
It is another object of the present invention to provide such an arrangement that passage positions of a 0th-order diffracted light component and (xc2x1) primary diffracted light components within a Fourier transfer surface for mask patterns in the projection optical system are set as arbitrary positions symmetric with respect to the optical axis of the projection optical system.
To accomplish the objects described above, according to one aspect of the present invention, there is provided, in the illumination optical system, a luminous flux distributing member such as a prism, etc. for distributing the illumination light into at least four luminous fluxes penetrating only a predetermined region on the Fourier transform surface for the mask patterns.
According to another aspect of the present invention, there is provided a movable optical member such as a movable mirror or the like in the illumination optical system to concentrate the luminous fluxes in predetermined positions on the Fourier transform surface for the mask patterns. The movable optical member is drivable to cause at least two beams of illumination light to pass through only the predetermined region on the Fourier transform surface with time differences from each other.
According to still another aspect of the present invention, there are provided the luminous flux distributing member or the movable optical member between an optical integrator such as a fly eye lens, etc. and the mask or between the light source and the optical integrator.
According to a further aspect of the present invention, the optical integrator is divided into a plurality of optical integrator groups which are set in discrete positions eccentric from the optical axis. At the same time, the illumination light is focused on the plurality of optical integrator groups, respectively.
According to still a further aspect of the present invention, the luminous flux distributing member is movable and exchangeable. The position in which the luminous flux passes above the Fourier transform surface for the mask patterns is arbitrarily set.
According to yet another aspect of the present invention, in a method of effecting the exposure while deviating a substrate position in the optical-axis direction of the projection optical system from an image forming surface of the mask patterns, the exposure is performed by making the illumination light incident on the mask at an inclined angle.
In accordance with the present invention, it is possible to actualize a projection type exposure apparatus exhibiting a higher resolving power and larger focal depth than in the prior art even by employing the ordinary reticle. Further, although the effect of improving the resolving power competes with a phase shifter, the conventional photo mask can be used as it is. It is also feasible to follow the conventional photo mask inspecting technique as it is. Besides, when adopting the phase shifter, the effect of increasing the focal depth is obtained, but it is hard to undergo influences of a wavefront aberration due to defocus even in the present invention. For this reason, a large focal depth (focal tolerance) is obtained.
Other objects and advantages of the present invention will become apparent during the following discussion taken in conjunction with the accompanying drawings.